1. Field of the Invention
The present invention relates to a method for matching antennas in a nuclear magnetic resonance imaging apparatus, such as a transmission/reception antenna or a reception antenna, to a high-frequency system connected to the antenna, and in particular to such a method wherein the matching ensues by setting a transformation network connected between the antenna and the high-frequency system.
2. Description of the Prior Art
Nuclear magnetic resonance imaging systems are used in medical diagnostics for producing tomograms of the human body. To that end, high-frequency electromagnetic signals are generated as a reaction to the excitation of the atomic nuclei in the body, and these electromagnetic signals are evaluated after being received by an antenna or by a resonator. The received signals are low-power, and must therefore be processed and evaluated without losses insofar as possible. This means that the signal-to-noise ratio cannot be significantly deteriorated in the individual steps of the signal processing.
A critical point in the signal processing chain is the detuning of the resonator due to the patient under examination. Such detuning is essentially defined by the size of the patient. Mismatches occur due to the detuning, and thus undesired reflections occur at the interface between the resonator and the high-frequency system connected thereto. Optimum power matching is thereby also disturbed.
In commercially available nuclear magnetic resonance imaging systems, the resonators are matched to the patient to avoid signal losses, or to reduce such losses to an uncritical level. A tuning path is provided for this purpose in addition to the transmission path and the reception path, with the impedance of the resonator being varied via this tuning path by means of variable capacitors so that optimum matching conditions are again present when a tomogram is produced. This tuning path constitutes a considerable portion of the overall electronics.
A magnetic resonance imaging apparatus is disclosed in German OS 37 28 863 wherein mismatches of a transmission/reception antenna caused by a patient are leveled. The reflection factor is continuously identified with a directional coupler during operation, and is forwarded as an actual value to a regulator. By comparing this reflection factor as the actual value with a rated (desired) value of zero for the reflection factor, the regulator forms a manipulated variable which is used to vary the capacitances in a transformation network. This control method is especially suitable for continuous wave (cw) mode. A change in the impedance during the measurement, however, is disadvantageous given certain pulse sequences and causes artifacts in the tomogram. Moreover, the method cannot be applied to a pure reception antenna.
European Application 0 114 405 discloses a circuit for undertaking automatic tuning of the reception antenna, the automatic tuning being implemented before a tomogram is produced. To that end, a plurality of transmission/reception cycles are conducted in a first disclosed method. First, a slice of the patient is excited with a transmission signal, and the amplitude of the reception signal following thereupon is stored. This procedure is repeated several times, with the transmission/reception antenna being detuned step-by-step by a tuning circuit before each transmission/reception cycle. The tuning at which the maximum reception signal occurs is also used for generating the tomogram. In a second method, a reception antenna is tuned without implementing an NMR transmission/reception cycle. A high-frequency signal is directly supplied to the reception circuit. After step-by-step detuning, the setting at which the largest signal amplitude occurred is selected. In both methods, only the amplitude changes due to the changes in the resonant properties of the antenna are interpreted for tuning, without taking the matching of the antenna to the impedance of the high-frequency system connected thereto into consideration. Despite antennas tuned to resonance, reflections and losses can still occur due to the non-matched impedances. In addition, the patient is stressed in the first tuning method by the high-power, high-frequency transmission pulses.